Field of the Invention
The invention relates to a method and a device for a transmit power control for connections between a base station and mobile stations of a radio communications system, for example a digital mobile radio network.
Radio communications systems serve to transmit subscriber signals via an air interface or radio interface using radio-frequency oscillations. Such radio communications systems are, for example, mobile radio networks or devices for wirelessly linking subscribers to a landline network or fixed network. The subscriber signals of various subscribers, that is to say mobile stations, are transmitted via a common radio interface. The term multiuser telecommunication system is used in this case. Time division multiplex, frequency division multiplex or code division multiplex methods can be used to separate the subscribers, that is to say the different connections between mobile stations and a base station. It is also possible to combine these methods.
In the following, a radio communications system is described in which information is transmitted to a plurality of mobile stations simultaneously in one frequency channel. These connections therefore cannot be separated by time division multiplex methods, nor by frequency division multiplex methods. The transmission from the mobile stations to the base station is designated as the upward direction, and the transmission from the base station to the mobile stations is designated as the downward direction.
The receivers of the base station or the mobile stations need to evaluate the information from a plurality of connections which is transmitted simultaneously in one frequency channel. The problem arises in this case that the received powers of the various connections vary greatly in the case of a constant transmit power due to the different distances between receiving and transmitting radio stations. In particular, the receivers, in which the received power of their connection is low compared to the rest of the received powers, have difficulties detecting data.
It is known to introduce a transmit power control into radio communications systems in order to overcome this problem. Thus, for example, it is known from the GSM mobile radio network (Global System for Mobile Communication) to set the transmit power both in the upward direction and in the downward direction in such a way that an adequate received power is available for the respective connection in the receiver. However, in the case of the GSM mobile radio network only information of a single connection is transmitted in one frequency channel and at one instant. The problem of jointly correcting or controlling the transmit powers for a plurality of connections therefore does not arise.
Published European patent application EP 0 565 505 A2 discloses a transmit power control in a CDMA (Code Division Multiple Access) system, the signal strength received by the base station being used to increase or reduce the transmit power of signals from the mobile station to the base station, and the transmit power of the signals from the base station to the respective mobile station.
Transmit power control is also carried out in the case of CDMA (Code Division Multiple Access) mobile radio systems, for example in accordance with the Mobile Radio Standard IS-95. In this case, the transmit powers for mobile stations for transmitting in the upward direction are set in such a way that constant received powers are present for the information of all connections in the case of the receiver of the base station. For this purpose, the signal/noise ratio is determined for each connection in the base station. The measurement results are signaled to the mobile stations, whereupon the latter set their transmit powers.
The receivers of the mobile stations need to evaluate the information only for their own connection. For this reason, the requirements placed on the quality of resolution or discrimination during a reception are less stringent. However, if the number of mobile stations in the radio cell of a base station increases, the number of connections between mobile stations and the base station also increases, with the result that the sum of the influences of the remaining connections increases compared to the received power of the own dedicated connection at the receiver of a mobile station. The mobile station becomes increasingly sensitive to received power fluctuations of its own connection.